Lambda calculus interpreter in common lisp

lambda.lisp

(defun interpret-lambda (expr &optional env)
  (etypecase expr
    (symbol (let ((res (assoc expr env))) (if res (cdr res) (error "Unbound symbol"))))
    (list (case (car expr)
            (lambda (list (car (second expr)) (third expr) env))
            (otherwise
             (let ((rand (interpret-lambda (second expr) env))
                   (rator (interpret-lambda (first expr) env)))
               (interpret-lambda
                (cadr rator)
                (cons (cons (car rator) rand) (third rator)))))))))
                
                
CL-USER> (interpret-lambda '(((lambda (x) (lambda (y) x))
                              (lambda (xbound) xbound))
                             (lambda (ybound) ybound)))
                             
(XBOUND XBOUND NIL)   ;; ad-hoc lambda form